1. Field of the Invention
This invention relates to a motor control system to be used for regulating the speed of a variable speed motor, and is more particularly directed to a system for controlling the speed of a scan motor as used in a variable magnification ratio photocopier.
2. Description of the Prior Art
In conventional motor control systems the measured speed of the motor is compared with the desired speed of the motor and the feedback signal directly proportional to this difference in speed is applied to force the motor to approach the target speed. Although this form of motor control is adequate for most uses, it is incapable of providing the close control of the motor speed which is required for driving a movable scanning system in a variable magnification ratio photocopier.
The typical variable magnification ratio photocopier includes a movable scanning system, a lens system, and a rotable photosensitive drum. The scanning system cooperates with the lens system to transmit an image of an original document to the surface of the photosensitive drum. To obtain an enlarged or reduced copy of this original document, the lens system must first be correctly positioned with respect to the scanning system, and then the speed of the scanning system must be adjusted to the correct speed ratio with respect to the speed of the photosensitive drum. Once the scanning system has been accelerated from rest to the scanning speed, close control over the scanning speed must be maintained to achieve an accurate photocopy.
This is achieved in prior art photocopiers by providing mechanical linkage between the scanning system and the rotable photosensitive drum surface. This prior art approach suffers from two defects. First, it is difficult to provide more than a small number of discrete magnification ratios when utilizing a mechanical system. Secondly, the optical performance of the photocopier deteriorates as the mechanical systems wear which necessitates periodic maintenance.